Correction of boundary artefacts in image data processing

ABSTRACT

An image processing system in which sets of image elements having display values outside of a target range B of display values are each respectively morphologically dilated. The intersection between the morphologically dilated sets of image elements is then identified and those image elements within the intersecting region are removed from the set of image elements having the target range B of display values. This removes image elements incorrectly appearing to have display values corresponding to the target range B of display values due to aliasing effects between regions of image elements having display values either side of the target range B of image values. The imaging may be two-dimensional or three-dimensional imaging. The morphological dilatation is preferably performed with a quasi-circular or a quasi-spherical structuring element having a radius of between two and three voxels.

FIELD OF THE INVENTION

This invention relates to the field of image data processing. Moreparticularly, this invention relates to the display of image elementshaving display values lying within a range of display values locatedwithin a broader overall range of display values.

BACKGROUND ART

It is known to provide image display systems in which image elements,whether they are representing a two-dimensional image or athree-dimensional image, are displayed to a user with a selection andmodified processing of at least some of the image elements being made independence upon the display values for those image elements. In certaincases, these image elements are directly related to a physicalcharacteristic of an object through the measurement of signal strengthvalues from a defection device. As an example, in medical imagingapplications signal values representing the signals returned from CATscanning or MRI scanning may be displayed with display elements havingtheir intensity or colour controlled by the value of the signal returnedfor each particular image element. In order to improve the ease ofinterpretation of such images it is known to map different colours todifferent ranges of display value such that particular features, e.g.blood vessels, may be made more visible within the image.

A particular problem arises when it is desired to selectively process ordisplay those image elements falling within a range of display valuesthat is itself located within a broader full range of display values forthe image as a whole. FIG. 1 of the accompanying drawings illustratessuch an image. The image contains a region 2 of image elements havinghigh display values, such as a region of bone within a medical image.This region 2 having high display values is located within an overallbackground 4 of image elements having low display values, such as imageelements representing soft tissue. Also present within this image is ablood vessel 6 containing a contrast enhancing agent that is representedby image elements having display values located somewhere between thehigh values of the bone region 2 and the low values of the backgroundsoft tissue region 4.

FIG. 2 of the accompanying drawings illustrates the problem that occursdue to the finite resolution of real imaging systems in that at theinterface between the bone region 2 and the soft tissue 4 there is aboundary formed of pixels having values lying somewhere between the highvalues of the bone region 2 and the low values of the soft tissue region4. In many cases, these boundary pixels will have display values closelysimilar to the display values corresponding to the blood vessel 6.Accordingly, if the image of FIG. 2 is processed to enhance theappearance, or otherwise select in some way, the display elements havingdisplay values corresponding to the blood vessels 6, then this willerroneously also highlight a region at the interface between the boneregion 2 and the soft tissue 4 that due to aliasing effects appears tohave the appropriate display value.

SUMMARY OF THE INVENTION

Viewed from one aspect the present invention provides a method ofprocessing image data formed of an array of image elements, each imageelement having a display value, said method comprising the steps of:

identifying a set of target image elements having display values withina target range of display values from B_(min) to B_(max);

identifying a set of low display value image elements having displayvalues below T_(min);

identifying a set of high display value image elements having displayvalues above T_(max);

applying a morphological dilation to said set of low display value imageelements to generate a dilated set of low display value image elements;

applying a morphological dilation to said set of high display valueimage elements to generate a dilated set of high display value imageelements;

identifying an intersection set of image elements that are present inboth said dilated set of low display value image elements and saiddilated set of high display value image elements; and

removing from said set of target image elements any image elements alsopresent within said intersection set of image elements to form amodified set of target image elements.

The invention recognises that the image elements that are incorrectlyappearing to have display values corresponding to the target range ofdisplay values occur at the interfaces between regions of image elementswith display values at opposite sides of the target range and if theseinterface image elements can be reliably identified, then they may beremoved. The invention identifies such interface image elements bymorphologically dilating the regions (i.e. sets of image elements) ateither side of the target region and then determining the intersectionof those dilated regions. The dilated regions will intersect where theytouch one another and accordingly this technique selectively identifiesthe interface regions rather than picking up image elements that lie atan interface between the target region and a region of image elementshaving display values outside of the target range. When these interfaceimage elements have been identified, they may be removed from the set ofimage elements having a display value within the target range and themodified set of image elements having the target display value can thenbe displayed with reduced interface aliasing artefacts or in some otherway processed, e.g. volume measured.

The technique of the invention is able to identify and remove the imageelements producing the erroneous artefacts within the image withoutsignificantly impacting the display of image elements that are correctlyidentified. By comparison, a technique that merely morphologicallyexpanded either the high display value region or the low display valueregion to overwrite any interface image elements would also be likely tooverwrite the edges or the fine detail in the display of the regionsimage elements that correctly have the target range of display values,e.g. fine capillaries may be lost from the image.

It will be appreciated that the technique described above could beapplied to both two dimensional images and three dimensional images. Theinvention is particularly well suited to use within imaging arrays ofthree dimensional voxel data as such images can be significantlydegraded by the aliasing artefacts between regions as discussed above.

The morphological dilatation could take a variety of forms dependingupon the particular circumstances, but preferably has the form of aspherical morphological dilatation whereby each voxel is projected ontoall the points within a quasi spherical surrounding region.

The spherical structuring element used in this morphological dilatationcould have a variety of sizes, but the invention has been found to beparticularly effective when the structuring element has a radius ofbetween 2 and 3 voxel sizes, and more preferably substantially 2.5 voxelsizes.

It will be appreciated that the image elements have associated displayvalues that correspond to the ranges being identified and are used tocontrol the way in which those image elements are displayed. Inpreferred practical systems, the display values do not correspond towhat would normally be regarded as visual properties, such as colour orintensity, but instead relate to detected signal values from measuringsystems such as CAT scanners, and MRI scanners, ultrasound scanners andPET systems.

The invention is particularly well suited to the removal of artefactsfrom images when attempting to discriminate between blood vesselscontaining contrast enhancing agents, soft tissue and bone within amedical diagnostic, such as angiogram, image.

Other aspects of this invention provide apparatus for processing imagedata and a computer program for controlling a computer to process imagedata in accordance with the above described techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 schematically illustrates an image having regions correspondingto different display values;

FIG. 2 schematically illustrates the image of FIG. 1 showing thealiasing artefacts that can arise;

FIG. 3 schematically illustrates the selection of a target range ofdisplay values to highlight within an image;

FIG. 4 is a flow diagram illustrating a technique for removing artefactsfrom an image such as that shown in FIG. 2;

FIG. 5 is a mathematical representation of the processing of FIG. 4;

FIG. 6 illustrates an image both before and after the removal of theartefacts described above; and

FIG. 7 schematically illustrates a general purpose computer of the typethat may be used to carry out processing in accordance with the abovetechniques.

DETAILED DESCRIPTION

Image data, such as a collection of say 100 two-dimensional 512*512images collected from a CAT scanner, a MRI scanner, an ultrasoundscanner or a PET (Positron emission tomography system), may be subjectto image processing in accordance with known techniques to produce athree-dimensional representation of the structure imaged (various userselected two-dimension projections of the three-dimensionalrepresentation are typically displayed on a computer monitor). Thetechniques for generating such three-dimensional representations ofstructures from collections of two-dimensional images are known inthemselves and will not be described further herein.

In order to increase the understandability of the three-dimensionalrepresentations generated it is known to select ranges of display valuesfor highlighting or selective processing in some other way. As anexample, voxels having a particular range of display value may be tintedwith a vivid colour to stand out within the image or may be selected forremoval from the image to reveal other more interesting features.

FIG. 3 is a histogram illustrating the frequency of occurrence of voxelswithin an image as a function of the signal value associated with thosevoxels. As illustrated in FIG. 3, there is a target range B of interestthat may have its upper B_(max) and lower B_(min) boundaries selected bythe user. The user may select this range to try to pick out bloodvessels containing a contrast enhancing agent, such as maybe desiredwhen performing angiography. This target region is positioned within thebroader full range of signal values that may be generated with a lowersignal value range A bounding the target range B on its lower side.Similarly, a higher value range C bounds the target range B on itshigher side. As previously discussed, a problem can arise when tissuethat returns a signal value within the range A abuts tissue that returnsa signal value within the range C as the spatial resolution of thesystem and the consequences of finite resolution sampling may result inan image element, voxel, being generated at this interface with a signalvalue corresponding to the range B even though this is not truly a bloodvessel region that it is desired to highlight.

FIG. 4 is a flow diagram illustrating the artefact removal technique. Atstep 10, the voxels that are to form the image are captured with eachone having a corresponding display value, in this case the displayvalues are representative of signal strength values from a detectiondevice, and indicate a density value D. At step 12, the set of voxelshaving a density within the user specified target range B of densityvalues illustrated in FIG. 3 is identified. At step 14, the set ofvoxels having a density corresponding to the range C of FIG. 3 isidentified. At step 16, the set of voxels having a density correspondingto the range A of FIG. 3 is identified.

The sets of voxels identified at steps 14 and 16 are then subject torespective spherical morphological, dilatation to produce dilated setsof voxels. The morphological dilatation can take a variety of forms, butin this example uses a quasi-spherical structuring element based upon avoxel approximation to a sphere having a radius of between 2 and 3voxels, but preferably substantially 2.5 voxels. The morphologicaldilatation with such a spherical structure acts to project each voxelvalue onto all of the voxels within a region surrounding the startingvoxel defined by the spherical structure. This slightly expands/dilatesthe region concerned.

At step 20, the two dilated sets of image elements are compared toidentify image elements appearing within both sets. These image elementscorrespond to the interface regions between the two sets. Theseinterface regions are the place where voxels incorrectly aliased so thatthey appear to be within the target range B may appear. Accordingly, theset of target voxels identified at step 12 is compared with thisintersecting set of voxels identified at step 20 and any voxels thatappear in both sets are removed from the set of voxels identified atstep 12. This occurs at step 22. At step 24, the resulting set of targetvoxels having the interface artefact removed from them are displayed.

FIG. 5 is a mathematical representation of the processing that occurs inaccordance with the flow diagram of FIG. 4.

FIG. 6 illustrates a Before image in which the artefact is present andan After image in which the artefact is not present. Each image containsa region of bone 26, blood vessels 28 and a soft tissue region 30. Inthe Before image, the interface between the soft tissue region 30 andthe bone 26 aliases to display values similar to those of the bloodvessels 28 and accordingly this interface region is tinted in the sameway as the blood vessel 28 making the image more difficult to interpret.The After image shows the effect of applying the above technique toremoving this artefact. This artefact is removed without inappropriatelychanging the shape of the regions appearing within the image and withoutthe possibility of obliterating fine detail that may occur by simplydilating one region outside of the target range.

FIG. 7 schematically illustrates a general purpose computer 132 of thetype that may be used to perform processing in accordance with the abovedescribed techniques. The computer 132 includes a central processingunit 134, a read only memory 136, a random access memory 138, a harddisk drive 140, a display driver 142 and display 144 and a userinput/output circuit 146 with a keyboard 148 and mouse 150 all connectedvia a common bus 152. The central processing unit 134 may executeprogram instructions stored within the ROM 136, the RAM 138 or the harddisk drive 140 to carry out processing of data values that may be storedwithin the RAM 138 or the hard disk drive 140. Data values may representthe image data described above and the processing may carry out thesteps illustrated in FIG. 4 and as expressed mathematically in FIG. 5.The program may be written in a wide variety of different programminglanguages. The computer program itself may be stored and distributed ona recording medium, such as a compact disc, or may be downloaded over anetwork link (not illustrated). The general purpose computer 132 whenoperating under control of an appropriate computer program effectivelyforms an apparatus for processing image data in accordance with theabove described technique. The general purpose computer 132 alsoperforms the method as described above and operates using a computerprogram product having appropriate code portions (logic) for controllingthe processing as described above.

1. A method of processing image data formed of an array of imageelements, each image element having a display value, said methodcomprising the steps of: identifying a set of target image elementshaving display values within a target range of display values fromB_(min) to B_(max); identifying a set of low display value imageelements having display values below B_(min); identifying a set of highdisplay value image elements having display values above B_(max);applying a morphological dilation to said set of low display value imageelements to generate a dilated set of low display value image elements;applying a morphological dilation to said set of high display valueimage elements to generate a dilated set of high display value imageelements; identifying an intersection set of image elements that arepresent in both said dilated set of low display value image elements andsaid dilated set of high display value image elements; and removing fromsaid set of target image elements any image elements also present withinsaid intersection set of image elements to form a modified set of targetimage elements.
 2. A method as claimed in claim 1, wherein said imageelements are voxels and said array is a three dimensional array ofvoxels.
 3. A method as claimed in claim 2, wherein said morphologicaldilation applied to said set of low display value image elements uses aquasi-spherical structuring element.
 4. A method as claimed in claim 2,wherein said morphological dilations applied to said set of low displayvalue image elements and said set of high display value image elementsuse quasi-spherical structuring elements.
 5. A method as claimed inclaim 3, wherein said quasi-spherical structuring element has a radiusof between 2 and 3 voxels.
 6. A method as claimed in claim 5, whereinsaid quasi-spherical structuring element has a radius of substantially2.5 voxels.
 7. A method as claimed in claim 1, wherein said displayvalues represent a measured signal strength value returned from aportion of a subject being imaged mapped to a corresponding imageelement.
 8. A method as claimed in claim 7, wherein said measured signalstrength values are detected using one of: CAT scanning; and MRIscanning; ultrasound scanning; and PET.
 9. A method as claimed in claim1, wherein said set of low value image elements correspond to softtissue, said set of high value image elements correspond to bone andsaid target set of image elements correspond to blood vessels containinga contrast enhancing agent.
 10. Apparatus for processing image dataformed of an array of image elements, each image element having adisplay value, said apparatus comprising: a target set identifieroperable to identify a set of target image elements having displayvalues within a target range of display values from B_(min) to B_(max);a low display value set identifier operable to identify a set of lowdisplay value image elements having display values below B_(min); a highdisplay value set identifier operable to identify a set of high displayvalue image elements having display values above B_(max); a low setdilator operable to apply a morphological dilation to said set of lowdisplay value image elements to generate a dilated set of low displayvalue image elements; a high set dilator operable to apply amorphological dilation to said set of high display value image elementsto generate a dilated set of high display value image elements; anintersection identifier operable to identifying an intersection set ofimage elements that are present in both said dilated set of low displayvalue image elements and said dilated set of high display value imageelements; and an intersection remover operable to remove from said setof target image elements any image elements also present within saidintersection set of image elements to form a modified set of targetimage elements.
 11. Apparatus as claimed in claim 10, wherein said imageelements are voxels and said array is a three dimensional array ofvoxels.
 12. Apparatus as claimed in claim 11, wherein said morphologicaldilation applied to said set of low display value image elements uses aquasi-spherical structuring element.
 13. Apparatus as claimed in claim11, wherein said morphological dilations applied to said set of lowdisplay value image elements and said set of high display value imageelements use quasi-spherical structuring elements.
 14. Apparatus asclaimed in claim 12, wherein said quasi-spherical structuring elementhas a radius of between 2 and 3 voxels.
 15. Apparatus as claimed inclaim 14, wherein said quasi-spherical structuring element has a radiusof substantially 2.5 voxels.
 16. Apparatus as claimed in claim 10,wherein said display values represent a measured signal strength valuereturned from a portion of a subject being imaged mapped to acorresponding image element.
 17. Apparatus as claimed in claim 16,wherein said measured signal strength values are detected using one of:CAT scanning; MRI scanning; ultrasound scanning; and PET.
 18. Apparatusas claimed in claim 10, wherein said set of low value image elementscorrespond to soft tissue, said set of high value image elementscorrespond to bone and said target set of image elements correspond toblood vessels containing a contrast enhancing agent.
 19. A computerprogram product comprising a computer-readable medium containingcomputer program code operable to control a computer to process imagedata formed of an array of image elements, each image element having adisplay value, said computer program code comprising: target setidentifying logic operable to identify a set of target image elementshaving display values within a target range of display values fromB_(min) to B_(max); low display value set identifying logic operable toidentify a set of low display value image elements having display valuesbelow B_(min); high display value set identifying logic operable toidentify a set of high display value image elements having displayvalues above B_(max); low set dilating logic operable to apply amorphological dilation to said set of low display value image elementsto generate a dilated set of low display value image elements; high setdilating logic operable to apply a morphological dilation to said set ofhigh display value image elements to generate a dilated set of highdisplay value image elements; intersection identifying logic operable toidentifying an intersection set of image elements that are present inboth said dilated set of low display value image elements and saiddilated set of high display value image elements; and intersectionremoving logic operable to remove from said set of target image elementsany image elements also present within said intersection set of imageelements to form a modified set of target image elements.
 20. A computerprogram product as claimed in claim 19, wherein said image elements arevoxels and said array is a three dimensional array of voxels.
 21. Acomputer program product as claimed in claim 20, wherein saidmorphological dilation applied to said set of low display value imageelements uses a quasi-spherical structuring element.
 22. A computerprogram product as claimed in claim 20, wherein said morphologicaldilations applied to said set of low display value image elements andsaid set of high display value image elements use quasi-sphericalstructuring elements.
 23. A computer program product as claimed in claim21, wherein said quasi-spherical structuring element has a radius ofbetween 2 and 3 voxels.
 24. A computer program product as claimed inclaim 23, wherein said quasi-spherical structuring element has a radiusof substantially 2.5 voxels.
 25. A computer program product as claimedin claim 19, wherein said display values represent a measured signalstrength value returned from a portion of a subject being imaged mappedto a corresponding image element.
 26. A computer program product asclaimed in claim 25, wherein said measured signal strength values aredetected using one of: CAT scanning; MRI scanning; ultrasound scanning;and PET.
 27. A computer program product as claimed in claim 19, whereinsaid set of low value image elements correspond to soft tissue, said setof high value image elements correspond to bone and said target set ofimage elements correspond to blood vessels containing a contrastenhancing agent.
 28. A method as claimed in claim 4, wherein saidquasi-structural structuring elements have radii of between 2 and 3voxels.
 29. A method as claimed in claim 28, wherein saidquasi-spherical structuring elements have radii of substantially 2.5voxels.
 30. Apparatus as claimed in claim 13, wherein saidquasi-structural structuring elements have radii of between 2 and 3voxels.
 31. Apparatus as claimed in claim 30, wherein saidquasi-spherical structuring elements have radii of substantially 2.5voxels.
 32. A computer program product as claimed in claim 22, whereinsaid quasi-structural structuring elements have radii of between 2 and 3voxels.
 33. A computer program product as claimed in claim 32, whereinsaid quasi-spherical structuring elements have radii of substantially2.5 voxels.